BIOL 211Lecture Notes # 1UoN, CAS, DBSC
Water and Plant Cells
Water plays a crucial role in the life of plants.For every gram of organic matter absorbed by the plant, approx. 500 g of water is absorbed by the roots, transported through the plant body and lost to the atmosphere.Every plant must delicately balance its uptake and loss of water.
Cell walls allow plant cells to build up large internal hydrostatic pressures, called turgor pressure, which is a result of their normal water balance. Turgor pressure is essential for many physiological processes, including cell enlargement, gas exchange in the leaves, transport in the phloem, and various transport processes across membranes. Turgor pressure also contributes to the rigidity and mechanical stability of non lignified plant tissues.
Water in Plant Life
Water makes up most of the mass of plant cells.Each cell contains a large water-filled vacuole. The cytoplasm makes up only 5 to 10% of the cell volume; the remainder is vacuole.
Water typically constitutes 80 to 95% of the mass of growing plant tissues.Common vegetables such as carrots and lettuce may contain 85 to 95% water.Wood, which is composed mostly of dead cells, has lower water content.Seeds, with a water content of 5 to 15%, are among the driest of plant tissues.
Water is the most abundant and arguably the best solvent known. As a solvent, it makes up the medium for the movement of molecules within and between cells and greatly influences the structure of proteins, nucleic acids, polysaccharides, and other cell constituents.
Plants continuously absorb and lose water.Most of the water lost by the plant evaporates from the leaf as the CO2 needed for photosynthesis is absorbed from the atmosphere.Such water loss is called transpiration.In addition, the stream of water taken up by the roots is an important means of bringing dissolved soil minerals to the root surface for absorption
Water Transport Processes
When water moves from the soil through the plant to the atmosphere, it travels through a widely variable medium (cell wall, cytoplasm, membrane, air spaces).The mechanisms of water transport also vary with the type of medium.
Water can cross plant membranes by:
- diffusion of individual water molecules through the membrane bilayer.
- bulk flow of water molecules through a water-selective pore formed by integral membrane proteins such as aquaporins.
Aquaporins are integral membrane proteins that form water-selective channels across the membrane. Because water diffuses faster through such channels than through a lipid bilayer, aquaporins facilitate water movement into plant cells.
Although the presence of aquaporins may alter the rate of water movement across the membrane, they do notchange the direction of transport or the driving force for water movement.
Diffusion
Diffusion is the movement of molecules (particles) by random thermal agitation.Water molecules in a solution are not static; they are in continuous motion, colliding with one another and exchanging kinetic energy.The molecules intermingle as a result oftheir random thermal agitation. This random motion is called diffusion.Diffusion causes the net movement of molecules from regions of high concentration to regions of low concentration — that is, down a concentration gradient.
Thermal motion of molecules leads to diffusion — the gradual mixing of molecules and eventual dissipation of concentration differences.
Bulk Flow
Bulk flow is the concerted movement of groups of molecules en masse, most often in response to a pressure gradient.Pressure-driven bulk flow of water is the predominant mechanism responsible for long-distance transport of water in the xylem.In contrast to diffusion, pressure-driven bulk flow is independent of solute concentration gradients.
Osmosis
Osmosis is the movement of solvent molecules (water and other small uncharged substances) through a semi-permeable membrane.Movement against the concentration gradient (from low concentration. to high concentration).Membranes of plant cells are selectively permeable.
Chemical Potential of Water
All living things require continuous input of free energy to maintain and repair their organized structures, as well as to grow and reproduce.The chemical potential of water is a quantitative expression of the free energy associated with water.
Free energy = potential to perform work. The chemical potential is a relative quantity: difference between the potential of a substance in a given state and the potential of the same substance in a standard state.
Unit = energy per mole of substance (J mol-1)
Water potential: chemical potential of water/volume of 1 mol of water (18 x 10-6 m3 mol-1).
Water potential is a measure of the free energy of water per unit volume (J m-3).
The main driving force behind the movement of water is the difference between free energies of water molecules on two sides of the semi-permeable membrane.For non-electrolytes, free energy is known as chemical potential (psi, Ψ).For water, free energy is called as water potential (Ψw).
The major factors influencing the water potential in plants are:
•Concentration (solutes)
•Pressure
•Gravity
Water potential is symbolized by the letter Ψw (Greek letter psi)
Ψw = Ψs + Ψp + Ψg
* Solutes
Ψs:solute potential or osmotic potential.
It is the effect of solutes on water potential.Solutes reduce the free energy of water.Mixing solutes and water increases the disorder of the system, thereby lowering free energy.The osmotic potential is independent of the specific nature of the solute.
*Pressure
Ψp: hydrostatic pressure of the solution (pressure potential).
Positive pressures raise the water potential. Negative pressures reduce it.The positive pressure within cells is referred as Turgor pressure.Ψp can also be negative, as in the case of xylem and walls between cells. Water in the reference state is at ambient pressure, so Ψp = 0 MPa, in the standard state. Thus, Ψw” for pure water is 0 MPa, at 1 Atmosphere and a particular temperature.
Water potential is usually expressed in pressure units such as bars or atmospheres (1 bar = 0.987 atmospheres).Water potential is also expressed in unit of pressure called pascals (Pa);
1 Pa = 1 Newton/square meter (Nm-2)
1 MPa (one megapascal) = 10 bars = 9.87 atmospheres.
*Gravity
Gravity causes water to move downward unless the force of gravity is opposed by an equal and opposite force.Ψg depends on the height (h) of the water above the reference-state water, the density (pw) and the acceleration due to gravity (g)
Ψg = pwgh
At the cell level, Ψg is insignificant in comparison to Ψs and Ψp.
Therefore, the equation for Ψw is simplified to Ψw = Ψs + Ψp.
Water Potential in the Plant
Cell growth, photosynthesis, and crop productivity are all influenced by water potential and its components.Like the body temperature of humans, it is a good indicator of plant health.Water enters the cell along a water potential gradient.Water potential is lowered by the addition of solutes and because water potential value is zero for pure water, all other water potential values will be negative.
In osmotic or other systems, the movement of water will take place from a region of higher water potential (less negative) to a region of lower water potential (more negative).Water potential values of plant cells under different osmotic conditions are as follows:
In plasmolysed or flaccid cell:
Ψw (lowest) = Ψs (as Ψp = nil)
In partially turgid cell:
Ψw (higher) = Ψs + Ψp
In fully turgid cell:
Ψw (highest) = zero (as Ψp numerically equals Ψs but both having opposite signs).
Example:
Two cells, A and B, in contact with each other.Cell A has a pressure potential (turgor pressure) of 4 bars and contain cell sap with an osmotic potential of -12 bars.Cell B has a pressure potential of 2 bars and contains sap with osmotic potential of -5 bars. Then:
- Ψw of cell A = Ψs + Ψp
= - 12 + (+4)
= - 8 bars
- Ψw of cell B = Ψs + Ψp
= - 5 + (+2)
= - 3 bars
Hence, water will move from cell B to cell A (i.e. towards lower or more negative water potential).In an open osmotic system, (e.g. plasmolysed plant cells), the water potential and the osmotic potential values are numerically similar and also have the same sign (i.e. negative).In a closed osmotic system, (e.g. turgid plant cells), a pressure is imposed on water which increases the water potential, this pressure is called as turgor pressure.
A flaccid cell (in air) is dropped in the 0.1 M sucrose solution. Because the starting water potential of the cell is less than the water potential of the solution, the cell takes up water.After equilibration, the water potential of the cell rises to equal the water potential of the solution, and the result is a cell with a positive turgor pressure. Increasing the concentration of sucrose in the solution makes the cell lose water. The increased sucrose concentration lowers the solution water potential, draws water out from the cell, and thereby reduces the cell’s turgor pressure. In this case, the protoplast is able to pull away from the cell wall (i.e, the cell plasmolyzes) because sucrose molecules are able to pass through the relatively large pores of the cell walls.
Plasmolysis: contraction or shrinkage of the protoplasm from the cell wall. In a plasmolyzed cell, the space in between the cell wall and plasma membrane is filled with outer hypertonic solution.
Plant tissue in hypotonic solution (endosmosis); turgid cell; turgor pressure.
-Turgor Pressure (T.P.): a pressure exerted by the protoplasm against the cell wall and the cell becomes turgid.
-Plant tissue in hypertonic solution (Flaccid cell).
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Dr. Naima Bouhouche